Electrostatic atomization is a technique for dispersing matter, often as a fine plume of droplets from a liquid, by subjecting the matter to be atomized to a suitable electric field. A voltage is applied between an electrode proximal to the matter to be atomized (the spray electrode) and at least one other electrode in the vicinity of the spray electrode. Under suitable conditions, liquid in the electric field is broken up into a spray of substantially monodisperse particles. When a liquid meniscus is subject to such an electric field, the meniscus distorts into a Taylor cone from which a stream of droplets is emitted.
Common forms of electrostatic atomization in the art include so-called “point-to-plane” electrostatic atomization, where a target object to be atomized is charged to the opposite polarity of the liquid and becomes the counter-electrode or the discharge electrode itself. This configuration, exemplified in U.S. Pat. No. 7,150,412, allows all or the majority of liquid being atomized to arrive at and to coat the target as electrostatically atomized charged droplets follow the path of the electric field created between these two electrodes. Following the same principle, the target to be atomized may instead be earthed or grounded as disclosed in U.S. Pat. Nos. 4,801,086 and 3,735,925.
Alternatively, a configuration may comprise three or more electrodes so arranged that an electric field is created in between two or more electrodes within the spray device itself. Whilst there is some partial discharge of the liquid being atomized due to the proximity of a counter-electrode, the majority of charged droplets will leave the device and arrive at a non-predetermined target, for example in U.S. Pat. No. 6,302,331.
The size, charge and flow rate of droplets atomized from an electrostatic atomizer are in part determined by the physical properties of the material to be atomized and also the electric field strength at the site of atomizing. When material to be atomized, particularly a liquid, possesses appropriate physical properties of conductivity, viscosity and surface tension, a spray of particles with a substantially uniform distribution of charge and size may be achieved for a particular electric field present between the first and second electrodes. The particular electric field is typically achieved by applying a particular voltage between the first and second electrodes.
Since the electric field varies with electrode geometry, amongst other factors, the particular voltage will be dependent on the separation of the electrodes (i.e., the distance between the point of emanation of material from the spray device, which may be a spray electrode) and the second electrode (reference electrode). When, for example, a liquid composition is formulated to possess appropriate physical properties, the particular voltage may be required to be adapted to compensate for variation in the geometrical arrangement of spray electrode and the reference electrode, for example due to variation within manufacturing tolerances.
Alternatively, where there is variation in manufacturing tolerances of the liquid to be atomized such as may arise in batch-to-batch variation of the physical properties of the liquid, or in batch-to-batch variation of the physical properties of various kinds of drug raw materials, the particular voltage may require adapting in order to achieve a suitable spray.
It is desirable therefore to be able to monitor the conditions and performance of any electrostatic atomizer in order to achieve a suitable output of material from the device notwithstanding variation in geometrical arrangement of spray components, differences between formulation and batches of material to be atomized and changes in environmental conditions, which may affect properties of the matter to be atomized.
Further, regarding the spray of material, where an electrostatic atomizer comprises a reservoir for storing and delivering material to the site of spray, it is desirable to be able to determine the level of material in the reservoir and particularly so when the reservoir is empty or substantially empty. In this way, a user of the device can find a timing at which it is necessary to provide a replacement reservoir and energy is not wasted in attempting to spray material when there is nothing left to be atomized.
In respect of these needs to monitor spray conditions, a number of solutions have been disclosed in the art. For example, the device of WO2005/097339 provides a device comprising voltage- and current-monitoring circuits which monitor voltage applied to and current flowing between an emitter (or spray) electrode and a discharging “opposed” electrode. The device disclosed in US2009/0134249, measures discharge current between an atomizer electrode and counter electrode in order to establish that a suitable voltage has been applied between the electrodes for water condensate on the atomizer electrode to be dispersed by electrostatic atomization. The power supply of WO2007/144649 monitors the discharging current flowing through the first and second electrodes of the device and adapting the voltage applied between the electrodes in response. The electrostatic atomizer of WO2008/072770 monitors voltage “upstream” of the atomizer electrodes by virtue of an adaptation to a self-oscillation type DC/DC convertor.
These and other means for monitoring current and adapting the spray condition in response to variations in devices or ambient environmental conditions suffer from a disadvantage in that they detect discharge current between a first electrode (which is usually a spray electrode) and a second electrode (which is usually a discharge electrode) by measuring the current at the discharge electrode. In such cases it is necessary, that all, or a proportion of, the particles generated at the spray electrode are directed by an electric field applied between the electrodes towards the discharge electrode. In some cases, one or more addition electrodes or other means are employed to direct atomized particles such that the majority do not contaminate the discharge electrode and to avoid excessive wastage of material.
Inferential monitoring of electrostatic atomization by measurement of discharge current on the discharge electrode is inaccurate insofar as such monitoring relies on assumptions regarding the representative amount of charged material issued at the electrostatic spray site which reaches the discharge electrode. This amount is susceptible to, amongst other things, variations in device geometry, whether or not the matter to be atomized is present, the physical properties of the matter to be atomized, and ambient environmental conditions.
On the other hand, measurement of current flowing at the spray electrode would reflect the accurate value of current carried away by the charged particles, however it is impracticable for electrostatic atomizers as it would require accurate detection of very low current levels (1-100 μA typically drawn by the high voltage spray electrode) carried on a high voltage signal (typically several kV).
Often, a reservoir comprising material to be atomized is hidden from the user of an electrostatic atomizer and it is not immediately obvious as to the fill level of the reservoir, particularly if the electrostatic atomizer has been in use for some time. Various devices and methods for detecting, monitoring or measuring the level of a liquid, whether or not relating to an electrostatic atomizer, are known in the art. For example, in U.S. Pat. No. 5,627,522, the level of liquid in a reservoir is sensed by periodically lowering a pipette probe into the liquid and detecting a change in capacitance between the probe in the liquid and a probe in the air. Another known method is disclosed in EP 0887658, where the phase shift of electromagnetic waves reflected of the surface of liquid in a reservoir is compared to a reference, thereby providing information about the level of liquid left therein. The fill level of a reservoir may be inferred by counting doses such as disclosed in U.S. Pat. No. 6,796,303, until a preset number of doses have been reached and the device indicates an empty vessel. Such a system is unsuitable where the dose amount varies according to variations in performance of the device, for example due to changes in ambient environmental conditions. A similar technique is disclosed in U.S. Pat. No. 4,817,822. Another indirect method of monitoring the reservoir can be by the use of a flow measuring device. For example in WO 2008/142393 A1, such a device measures the pressure drop between a pair of spaced apart pressure sensors.